# std::bitset<N>::bitset

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< cpp‎ | utility‎ | bitset

bitset(); | (1) | (noexcept since C++11) (constexpr since C++11)
| (2) |
bitset( unsigned long val ); |  | (until C++11)
constexpr bitset( unsigned long long val ) noexcept; |  | (since C++11)
template< class CharT, class Traits, class Alloc > explicit bitset ( const std::basic_string<CharT, Traits, Alloc>& str, typename std::basic_string <CharT, Traits, Alloc>::size_type pos = 0, typename std::basic_string<CharT, Traits, Alloc>::size_type n = std::basic_string<CharT, Traits, Alloc>::npos, CharT zero = CharT('0'), CharT one = CharT('1') ); | (3) | (constexpr since C++23)
template< class CharT, class Traits > constexpr explicit bitset ( std::basic_string_view<CharT, Traits> str, std::size_t pos = 0, std::size_t n = std::size_t(-1), CharT zero = CharT('0'), CharT one = CharT('1') ); | (4) | (since C++26)
template< class CharT > explicit bitset( const CharT* str, std::size_t n = std::size_t(-1), CharT zero = CharT('0'), CharT one = CharT('1') ); | (5) | (since C++11) (constexpr since C++23)

Constructs a new bitset from one of several optional data sources:

1) Default constructor. Constructs a bitset with all bits set to zero.
2) Constructs a bitset from an unsigned integer val.
Given the number of bits in the value representation of unsigned long(until C++11)unsigned long long(since C++11) as S:

- The first (rightmost, least significant) std::min(S, N) bit positions are initialized with the corresponding bit values of val.
- If S is less than N, the remaining bit positions are initialized to zeroes.

3) Constructs a bitset using the characters in str. An optional starting position pos and length n can be provided, as well as characters denoting alternate values for set (one) and unset (zero) bits. `Traits::eq()` is used to compare the character values.
The effective length of the initializing string is std::min(n, str.size() - pos).
4) Similar to (3), but uses a std::basic_string_view instead of a std::basic_string.
5) Similar to (3), but uses a const CharT* instead of a std::basic_string.

Equivalent to bitset(n == std::basic_string<CharT>::npos ? std::basic_string<CharT>(str) : std::basic_string<CharT>(str, n), 0, n, zero, one). | (until C++26)
Equivalent to bitset(n == std::basic_string_view<CharT>::npos ? std::basic_string_view<CharT>(str) : std::basic_string_view<CharT>(str, n), 0, n, zero, one). | (since C++26)

### Parameters

val | - | number used to initialize the bitset
str | - | string used to initialize the bitset
pos | - | a starting offset into str
n | - | number of characters to use from str
zero | - | alternate character for unset bits in str
one | - | alternate character for set bits in str

### Exceptions

3,4) std::out_of_range if pos > str.size(), std::invalid_argument if any character is not one or zero.
5) std::invalid_argument if any character is not one or zero.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_constexpr_bitset` | `202207L` | (C++23) | A more constexpr `std::bitset`, overloads (3,5)
`__cpp_lib_bitset` | `202306L` | (C++26) | Interfacing `std::bitset` with std::string_view, (4)

### Example

Run this code

    #include <bitset>
    #include <climits>
    #include <iostream>
    #include <string>

    int main()
    {
        // empty constructor (1)
        std::bitset<8> b1; // [0,0,0,0,0,0,0,0]

        // unsigned long long constructor (2)
        std::bitset<8> b2(42);          // [0,0,1,0,1,0,1,0]
        std::bitset<70> bl(ULLONG_MAX); // [0,0,0,0,0,0,1,1,1,...,1,1,1] in C++11
        std::bitset<8> bs(0xfff0);      // [1,1,1,1,0,0,0,0]

        // string constructor (3)
        std::string bit_string = "110010";
        std::bitset<8> b3(bit_string);       // [0,0,1,1,0,0,1,0]
        std::bitset<8> b4(bit_string, 2);    // [0,0,0,0,0,0,1,0]
        std::bitset<8> b5(bit_string, 2, 3); // [0,0,0,0,0,0,0,1]

        // string constructor using custom zero/one digits (3)
        std::string alpha_bit_string = "aBaaBBaB";
        std::bitset<8> b6(alpha_bit_string, 0, alpha_bit_string.size(),
                          'a', 'B');         // [0,1,0,0,1,1,0,1]

        // char* constructor using custom digits (5)
        std::bitset<8> b7("XXXXYYYY", 8, 'X', 'Y'); // [0,0,0,0,1,1,1,1]

        std::cout <<   "b1: " << b1 << "\nb2: " << b2 << "\nbl: " << bl
                  << "\nbs: " << bs << "\nb3: " << b3 << "\nb4: " << b4
                  << "\nb5: " << b5 << "\nb6: " << b6 << "\nb7: " << b7 << '\n';
    }

Possible output:

    b1: 00000000
    b2: 00101010
    bl: 0000001111111111111111111111111111111111111111111111111111111111111111
    bs: 11110000
    b3: 00110010
    b4: 00000010
    b5: 00000001
    b6: 01001101
    b7: 00001111

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 396 | C++98 | the values of the zero and one characters for overload (3) were ​0​ and 1 (which do not correspond to '0' and '1') | added parameters to provide values for these characters
LWG 457 | C++98 | S was CHAR_BIT * sizeof(unsigned long) for overload (2), but unsigned long is not guaranteed to use all its bits to represent its value | consider the number of bits of the value representation instead
LWG 2250 | C++98 | the behavior was undefined if pos > str.size() is true | always throws an exception in this case

### See also

set | sets bits to true or given value (public member function)
reset | sets bits to false (public member function)

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